The present invention refers to a method for reducing angular blur in radar pictures.
A radar works by radiating electromagnetic waves from an antenna, and detecting the reflected waves (the echoes) from objects close to the radar. Very often, the radar can distinguish echoes from objects at different distances and thus has a range resolution. Often, the range measurement is discrete, in the sense that the measured range values are not continuously variable, but rather take one of a predetermined set of values, very often equidistant. The following discussion is limited to such radars, with a discrete-valued range measurement (range bins).
To survey a large area, many search radars have a scanning antenna, very often rotating to cover all directions, or going back and forth to cover a sector, say in azimuth. In this case, the radar receives in each angular position echoes from all studied range bins (in some range bins, there may be no echo). If the radar is pulsed, the detection is made only in discrete angular positions, one for each pulse.
In this case, the received amplitude (or power) of the radar echoes can be illustrated as a radar picture with a finite set of radar picture elements (pixels). In most cases, the radar picture of a cartesian sector is presented as a rectangle in polar coordinates, with a finite number of angular positions along the horizontal axis and a finite number of range bins along the vertical axis (this is called a “B scope” presentation). For convenience, very often the logarithm of the amplitudes (powers) are presented (deciBel scale, dB).
The following discussion is limited to radar with data presented as such rectangular B scope pictures with a finite number of pixels. The angular coordinate is generally thought of as azimuth, radar antenna scanning about a vertical axis, but elevation is also possible.
The radar sensitivity is greatest in a direction determined by the antenna. In most cases, this direction is in front of the antenna. Since the physical antenna width is a limited number of wavelengths corresponding to the frequency used, the antenna main beam, where the sensitivity is greatest, is not infinitely narrow, but has a certain angular width. The radar receives an echo from an object not only when the antenna is pointed directly towards the object, but also when the antenna is pointed slightly to the right of the object, when the object is illuminated by the left part of the antenna main beam, and when the antenna is pointed slightly to the left of the object, when the object is illuminated by the right part of the antenna main beam. A small (point-shaped) radar reflector thus gives echoes during the whole angular extent of the radar main beam.
Since there are—for redundancy, for Doppler analysis, or for other reasons—generally several angular positions used (several pulses transmitted) per radar antenna main beam width, the effect is to make radar pictures blurred in the angular coordinate. This blur makes analysis of the radar picture more difficult. It becomes non-trivial to determine the angle to a radar target, even if the target is point-shaped, and comparison between the radar picture and a picture of the same area by another sensor with different characteristics (maybe another radar) becomes difficult.
A number of methods exist for reducing angular blur and some major lines will be discussed below.
According to a first major line the methods are based upon deconvolution. Since the blur can be considered the result of convoluting a blur-free picture with the antenna beam, it can be eliminated in theory by making a Fourier transformation of the picture, in the angular direction, dividing with the Fourier transform of the antenna diagram, and then performing an inverse Fourier transformation. The rationale is that Fourier transformation converts convolution to multiplication.
According to a second major line methods within general radar are discussed. Frequently, data such as those in FIG. 1 showing a measured radar picture of a Cartesian sector in dB scale with angular positions along the horizontal axis and range bins along the vertical axis are passed through a “target extractor” which often works by defining a “target” as a local maximum with amplitude exceeding a suitably defined threshold. Then, in a given range bin, the angular position of the target is defined as the position where the maximum is attained. A more exact angular position can be computed by fitting a suitable curve, such as a parabola with angle position as the independent variable, to the data near the local maximum, and compute the angular position of the maximum of the parabola fitted. In particular, it is not difficult to fit a parabola to the three points closest to the local maximum, with the highest amplitudes, in particular if the angular positions are equidistant. To fit a parabola to more than three points, a least squares computation can be used.
This procedure generally works well for tracking of a limited number of targets, but is not always practicable for de-blurring a whole picture, for presentation or for other purposes.
According to a third major line a moving radar is used. For airborne radars, advanced methods as Synthetic Aperture Radar (SAR, [LHMN Ch. 15]), in its simplest form “Doppler Beam Sharpening” [LHMN p. 260], use a moving radar with a radar radial velocity (Doppler) measurement, and advanced signal processing, to get a very sharp “picture” of the surveyed part of the ground (not always described as a rectangle in polar coordinates). In this specification LHMN is used as a short reference to Lacomme/Hardange/Marchais/Normant, Air and Spaceborne Radar Systems. An Introduction, ISBN 1-891121-13-8, SciTech Publishing 2001.
According to a fourth major line methods for reduction of angular blur presuppose a particular type of radar antenna, as a monopulse antenna or an array antenna.
Tracking radars are radars which can measure how much the radar antenna direction (the boresight) differs from a desired direction (the difference is generally called the error angle). Tracking radars are used for aiming guns, guiding missiles, keeping an antenna tracked on a communications satellite, etc. Generally a tracking radar uses a monopulse antenna [Sherman]. In this specification Sherman is used as a short reference for Sherman, Monopulse Principles and Techniques, ISBN 0-89006-137-8, Artech House 1984. A monopulse antenna can use at least two antenna patterns, a sum pattern with its maximum directivity in the antenna direction, and a difference pattern which is sensitive for targets slightly off the antenna direction. There is one difference pattern for every angular variable used (azimuth, elevation). FIG. 3 shows typical sum and antenna patterns ([Sherman p. 138]). In FIG. 3 the quantities (field strengths) have both positive and negative signs. FIG. 4 shows the absolute value of the difference pattern, compared to the sum pattern.
Some radars—often airborne multi-function radars—use a monopulse tracking antenna when surveying and searching. Then, the difference channel can be used in various ways to reduce the picture blur due to the main (sum) antenna beamwidth. Two kinds of established methods are indicated below. Both may be covered by the name “Monopulse Beam Sharpening”.
Let S and D indicate the sum and difference channel respectively. Then S−k-abs(D), with a suitable small positive constant k, is a pattern which is =S in the antenna direction (boresight), where D=0, but which is smaller than S off boresight, and thus narrower than S. Such a pattern can be used to reduce the angular blur in radar pictures. To avoid negative amplitude (power) values, a combination max(0, S−k-abs(D)) may be used.
A much better and more sophisticated method is to compute the monopulse error angle in every point in the picture and determine a corrected angular position as “angular position+error angle”. The power detected in a range bin in a certain angular position is not stored in the original angular position, but rather in the corrected angular position [LHMN pp. 205-206]:corrected angular position=angular position+error angle
This gives a very effective “beam sharpening” for display or for other purposes.
Generally, in the picture to be de-blurred, there are only a limited number of allowed angular positions (called “sub-beams” in [LHMN, p. 206]), and then the right hand side of the formula will have to be rounded to the nearest allowed position. The “storage” may be performed by addition to already stored values (especially when adding powers), or by using an operation of type max(new_value,already_stored_values). An alternative, which in some cases gives more exact results, is to distribute the magnitude (power) on the two nearest allowed angular positions, maybe by linear interpolation.
If the radar uses an array antenna with several independent transmitting or receiving elements special methods are also available.